In a wireless communication system including, e.g., an heterogeneous network as radio access network, control signals and user data signals may be separated into two distinct overlaid networks, a network of macro cells wherein each macro cell includes a macro base station (e.g. referred to as MeNB), and a network of small cell base stations (also referred to as SeNB) controlled by one macro base station. Such overlaid networks are also called control/user-plane separated networks or C/U-plane separated networks (including a control plane base station controlling a plurality of user plane base stations).
FIG. 1 shows the general structure of a wireless communication system having two distinct overlaid networks. The networks comprise a macro cell network including one or more macro cells, each including a macro base station (MeNB). FIG. 1 schematically shows a single MeNB 100. The macro cells operate in currently existing frequency bands, for example in the 2 Gigahertz frequency band, using currently standardized systems like LTE/LTE-A and also guarantee backwards compatibility for legacy user equipments (UEs or mobile stations), i.e., such UEs which just support the current standards. FIG. 1 further shows a small cell network comprising a plurality of small cell base stations (SeNB) 1041 to 1045 each operating within respective areas 1061 to 1065 (also referred to as coverage area) defining the small cells. The SeNBs 1041 to 1045 defining the small cells 1061 to 1065 may operate in frequency bands different from the frequency bands used in the macro cell network, for example in higher frequency bands, like the 3-5 Gigahertz band. The SeNBs 1041 to 1045 of the small cells 1061 to 1065 are controlled by the MeNB 100 are may be connected to the umbrella network (the MeNB 100) via respective backhaul links 1101 to 1105. FIG. 1 further shows a user equipment 112 receiving control signals from the MeNB 100 as is schematically depicted by arrow 114 and that communicates user data signals via one of the small cells as is depicted by arrow 116.
In wireless communication networks in general and also in the network shown in FIG. 1, energy savings and energy efficiency are of specific interest. For achieving such savings and efficiency, one or more of the SeNBs may be put to sleep or may be turned off when not in use. A UE cannot set up a communication with a sleeping SeNB, rather, it needs to connect for a communication directly with the MeNB 100. In the “ON” or “ACTIVE” state the SeNB is fully on and sends both user data to connected UEs and pilot symbols to enable new UEs to connect. The pilot symbols may be sent by the SeNBs to enable the UEs to differentiate the respective SeNBs. In the “OFF” or “SLEEP” state the SeNB is in a stand-by mode in which it can neither send nor receive any radio signals and consumes a non-negligible amount of power. The off or sleeping SeNB may be woken up by the MeNB 100, by appropriate signaling via the backhaul link 110. Problems arising with small cells being in the sleep mode will now be discussed with reference to FIG. 2 which shows the wireless system of FIG. 1. It is assumed that the small cell base stations (SeNBs) 1041, 1042 and 1045 are in the sleep mode, as is indicated by dashed circle indicating the respective small cells 1061, 1062 and 1065. The SeNBs 1043 and 1044 are active. In dense small cell deployments, putting unused small cells, like cells 1061, 1062, and 1065, to sleep provides benefits in terms of energy savings and reduced interference.
However, this also results in some problems. One problem is the small cell discovery. For UEs, like UE 112, it is a challenge to reliably discover sleeping small cells, because such cells either stop transmitting discovery signals or reduce the frequency of such a discovery signal transmission in order to save energy. In the absence of discovery signals, it becomes impossible for UEs to discover sleeping cells. For example, the UE 112 may not be aware that it is in the vicinity of the two sleeping small cells 1061 and 1062 if they are not transmitting discovery signals. Even though a reduced periodic transmission of discovery signals from a sleeping cell may improve discovery, the reliability of this process is low and involves a lot of energy on the part of both the small cell and the UE in order to improve discovery speed and reliability.
Another problem with sleeping cells is that it is not immediately clear which resources and capabilities should be activated in a discovered small cell when several options are available. In FIG. 2, the UE 112 may somehow discover one or both nearby sleeping cells 1061, 1062, however in both cases it may be suboptimal for the small cell to activate all its resources. The default approach of activating all resources and capabilities is suboptimal, since it can lead, at best, to an under utilization of the activated resources. For example, in the situation depicted in FIG. 2, the UE 112 is capable of operating in the frequency bands f1 and f2, however it cannot operate in the frequency band f3 that is also provided by the small cells 1061 and 1062. Thus, activating in either of the small cells 1061 and 1062 the frequency band f3 is not required for serving UE 112. Activating all resources may further result in a deterioration of the communication environment with regard to existing active communication links. When activating in either of small cells 1061 and 1062 all available frequency bands f1 to f3, an increased interference within an existing communication link may occur. In the situation depicted in FIG. 2, the already active small cell 1063 operates in the frequency band f1, so that activating all frequency bands in the sleeping cells 1061 and 1062, including frequency band f1, may lead to an undesired interference situation deteriorating the communication environment.
Yet another problem with regard to sleeping small cells is that the process of activating a sleeping small cell, its discovery and the acquisition of the proper system information to connect to an activated small cell may result in a long connection setup delay experienced by the UE 112 when trying to connect to a small cell that has just been activated from its sleep mode so that no quick connection setup is possible.
Several approaches have been proposed in conventional technology, for example in publications and standardization communities (see conventional-technology references [1] and [2]), to address the above referenced problems, however these problems mostly focus on addressing the problems of sleeping small cell discovery and may be grouped into three approaches.
The first approach may be referred to as an uplink-based signaling approach in accordance with which a sleeping cell monitors uplink transmissions by leaving its radio frequency (RF) receiving chain in the on state. Upon detecting some UE activity, the sleeping cell wakes up from the sleep mode and activates its transmission chain to start transmitting discovery signals. UEs in the vicinity can discover the small cell and initiate connection procedures. This approach may have some advantages as it supports autonomous small cell on/sleep behavior, however this comes with a number of disadvantages. One disadvantage is that the small cell needs to maintain its RF receiving chain activated, which compromises any potential energy savings in the sleep mode. Furthermore, this approach puts a lot of strain on the UE energy resources as the UEs need to transmit its signals frequently and on several frequency resources in order to improve the speed and reliability of triggering a nearby sleeping small cell to wake up.
Another approach known from conventional technology is referred to as a downlink-based signaling approach in accordance with which small cells which are in the sleep mode, periodically or in response to a trigger signal, transmit discovery signals to enable UEs to discover and initiate connection procedures. Upon discovery, subsequent procedures are performed to fully activate the sleeping small cell. Like the above described uplink-based signaling approach, also the downlink-based signaling approach has the advantage of supporting an autonomous small cell on/sleep behavior. However, like the previous approach, at the same time it suffers from the same drawbacks. In addition in a dense small cell deployment, the transmission of unique discovery signals from all small cell base stations, including those being in the sleep mode, significantly increases the search space for the UE which can then lead to discovery delays.
Yet another known approach is referred to as a location-based scheme which relies on previously stored information to estimate whether a UE is in the vicinity of a small cell. One approach relies on storing RF maps that correspond to various small cell locations and using measured or reported radio fingerprints from UEs to determine when the UE is in the vicinity of a small cell, as is for example described in conventional-technology reference [3]. Another approach relies on storing the actual locations of small cells and using geographic location reports from a UE to determine if there are any small cells in the vicinity of the UE, as is for example described in conventional-technology references [4], [5]. Both approaches may use an external entity, for example a macro base station, to wake up a sleeping small cell. In location-based schemes, the RF receiving and transmission chains of the sleeping small cells can be switched off completely, which maximizes the achievable energy savings. However, proper functioning of these schemes may use a training phase to obtain accurate reference data, which can cause disruptions to the service provided.
In addition to the above mentioned limitations, state of the art mechanisms for a small cell activation focus on making binary decisions on whether to wake up a sleeping cell or to leave it in the sleep mode. Very little attention is paid to the fact that a sleeping small cell and a UE may have many resources and capabilities which may use more complex decisions being made regarding the resource and the capabilities to activate a sleeping small cell for a communication with a target UE.
The above problems regarding the delayed connection setup between a UE and a small cell (that was not in the sleep mode) also occur in situations, where the UE needs to make a connection to a new small base station due to the time for the acquisition of the proper system information.